Degenerative and/or traumatic damage to skeletal joints or other locations within a patient's body may require surgical intervention. During such surgical intervention, it is often necessary to position and/or support a surgical component at a desired location relative to the surgical site. Surgical components may include implants, trial implants, drills, burrs, saws, lasers, thermal ablators, electrical ablators, retractors, clamps, cameras, microscopes, guides, and other surgical components. Surgical sites may include a hip joint, knee joint, vertebral joint, shoulder joint, elbow joint, ankle joint, digital joint of the hand or foot, jaw, fracture site, tumor site, and other suitable surgical sites. For example, damage to the articular cartilage of a skeletal joint can result in pain and restricted motion. Prosthetic joint replacement is frequently utilized to alleviate the pain and restore joint function. In this procedure, the damaged parts of the joint are cut away and replaced with prosthetic components. Typically a resection guide is used to guide a cutter such as a saw blade or burr to cut a desired portion of the bone to prepare a seating surface for a prosthetic component. The resection guide must be carefully positioned to guide the cut at the appropriate location.
Many surgical procedures are now performed with surgical navigation systems in which sensors detect tracking elements attached in known fixed relationship to objects in the surgical environment such as surgical instruments, implants, or patient body parts. The sensors information is transmitted to a computer that triangulates the position and orientation of the tracking elements within surgical navigation system coordinates. Thus, based on the fixed relationship between the objects and the tracking elements, the computer can resolve the position and orientation of the objects and provide position and orientation feedback for surgeon guidance.
An anatomical joint formed between two bones comprises three planes and three rotation axes relative to the joint line and each bone with respect to the other. In a knee joint, the planes are known as the medial/lateral, anterior/posterior, and proximal/distal planes while the rotation axes are known as the external rotation of the femur, extension plane rotation, and varus/valgus rotation. FIG. 1 illustrates a femur 1, a tibia 3, and various axes of the knee joint in the frontal plane. Femur 1 has an anatomic axis 2 coinciding generally with its intramedullary canal. It also has a mechanical axis 4, or load axis, running from the center of the femoral head to the center of the knee. An angle 6 between axes 2, 4 in the frontal plane varies within the patient population but is on the order of 4-9°. Axes 2, 4 are approximately superimposed in the sagittal plane (FIG. 2). Likewise, tibia 3 has a mechanical axis 5 coinciding generally with its intramedullary canal. Mechanical axis 5 of the tibia runs from the center of the knee to the center of the ankle. The knee flexes about a joint line 8.
FIG. 2 illustrates the knee joint from the side or sagittal view and various bone cuts that may be made to align implant components. A distal femoral cut 10 is typically made perpendicular to femoral axes 2, 4 in the sagittal plane. A proximal tibial resection 12 is typically cut to match the natural posterior slope, or rotation, 16 of the proximal tibia relative to mechanical axes 4, 5 and an axis 18 which is perpendicular to mechanical axis 5. The distance between distal femoral cut 10 and proximal tibial cut 12 along mechanical axes 4, 5 is the extension gap. The distal femur and proximal tibia are typically resected to be parallel to a joint line 8, and thus perpendicular to mechanical axes 4, 5 as indicated at 10 and 12. Other cuts may be made depending on the components that are to be implanted. These include an anterior femoral cut 20, an anterior femoral chamfer cut 22, a posterior femoral chamfer cut 24, and a posterior femoral cut 26. A patella cut 28 may also be made to allow for replacement of the patellar articular surface. In a unicondylar knee replacement, only the medial or lateral side of the knee joint is resurfaced. Furthermore, the trochlear, or patellar bearing, surface of the femur is typically left intact in a unicondylar procedure. Unicondylar implant designs vary, but typically only distal femoral cut 10, posterior femoral chamfer cut 24, and posterior femoral cut 26 are needed to accommodate the unicondylar femoral implant.
In many cases it is desirable to observe movement of the joint before determining whether to reset the femur and proximal tibia in typical fashion. For example, a surgeon may determine, based on observed articulation of the joint, bone defects, patient age and patient condition, that an atypical resection angle is desirable. The surgeon may also observe soft tissues such as tendons and ligaments and determine that soft tissue balance requires atypical resections. Resection angle variations as small as one degree may impact the function of the joint. Systems and methods are needed which enable determination of resection planes intraoperatively to achieve soft tissue balance.